I managed to fix my speedo today. It had been acting up for the last few thousand miles (odometer jumping ahead (10s and 100s) and then slipping, speedo bouncing, etc.). Finally, on Sunday, someplace in Kansas, the odometer stopped working completely, then the speedo started making load popping sounds and jumping (like the cable was being snapped). I disconnected the cable and drove home.
I took it apart today and replaced the broken plastic gear as well as the odometer wheels and "cams" from a spare speedo. I also "roughened" the shaft to keep the gear from slipping. Kind of a PITA, but I got is all back together and it seems to work well. We'll see how long it holds out.